Tomographical imaging techniques have been developed to enhance images produced by conventional X-ray imaging. X-ray tomography blurs out undesirable images of superimposed structure to accentuate images of principal interest. In linear tomography the radiation source and the detector film are moved in opposite directions. The patient is placed on a metal structure that rotates about a pivot point or fulcrum. This fulcrum plane remains in focus while all other planes are blurred. Linear tomography does not permit differentiation of soft tissue or provide quantitative information of attenuation properties of tissues.
Computerized axial tomography (CAT) scanners overcome the limitations of linear tomography by rotating the source with respect to the patient and a plurality of fixed detectors arranged around the patient.
The result is an x-ray image that gives the illusion of three dimensionality. Because the CAT beam is rotated around the body, it can image organs that overlap and are therefore obscured under conventional x-rays or radiograms. By using several hundred x-ray detectors to produce one exposure, the CAT is an order of magnitude more sensitive to slight gradations in density than radiographs, which frequently do not allow the practitioner to distinguish between tissues of approximately the same density.
Another useful imaging technique is Positron Emission Tomography (PET).
In PET, image construction is based on the location and intensity of gamma rays emitted in the region of a neutron-poor isotope. Planar images formed by computer PET result from the reconstruction of attenuation corrected coincident pairs of 511 KeV gamma rays mapped at the body surface from injected positron-emitting tracers. Neutron-poor isotopes undergo radioactive decay by the process P.sup.+ .fwdarw.N+e.sup.+ +.nu., where P.sup.+ is a nuclear proton, N is a neutron, e.sup.+ is a positron, an .nu. is a neutrino. A neutron-poor isotope, such as .sup.11 C will undergo beta decay, in which a proton becomes a neutron and a positron and a neutrino are ejected from the nucleus. Within a short distance, the positron encounters an electron, upon which the two annihilate each other and give rise to a pair of gamma-ray photons that depart at an angle of about 180.degree., each carrying an energy of 0.511 MeV.
The gamma photon derived from the decay of the isotopes within the patient's body is sensed by a circumferential array of collimated detectors, the circumference of which is designed so that opposite members of the ring are coupled. A signal is recorded only when both members of the detector pair sense coincidental photons. By using a slight time difference in the activation of the detectors, one can locate the source of the photons on the basis of time-of-flight differences from an eccentrically positioned emitter. Data are fed into a computer which generates the image based on location and source intensity. Tissue attenuation is taken into account. Spatial resolution is about 0.5 cm. Only minute amounts of tracers are needed. The radiation dose is small.
PET is particularly adapted to kinetic analysis of physiologic and body chemical events, including blood volume, blood flow, consumption of oxygen and substrates and the distribution and occupancy of surface and intracellular receptors. In addition to imaging the distribution of injected positron-emitting tracers, PET imaging provides a means for measuring the presence and amount of such important biological substances as oxygen contained in tissue induced by in vivo activation during irradiation with protons or by photoneutron production (O.sup.16 (.gamma.,n)O.sup.15) during radiotherapy of tumors. The importance of increasing the dose to hypoxic regions in tumors is well known in radiobiology and this would provide the first practical means for a non invasive on line characterization of tissue oxygen levels.
Single-Photon Emission Computed Tomography (SPECT) completes the spectrum of commonly used medical radionuclide or X-ray imaging devices.
The basic principles of SPECT are very similar to x-ray CT. A transverse section is divided into a matrix of small volume elements or voxels. The radioactivity of each voxel is computed from projection data obtained from a 180.degree. or full 360.degree. scan around the patient. The projection data are obtained by translating and rotating arrays of multiple detectors (multidetector approach) or by rotating one or more scintillation cameras around the patient (camera-based approach).
U.S. Pat. No. 4,833,327, incorporated herein by reference, describes a radioisotopic imaging system in which thin and thick electronic collimation detectors are arrayed about a radiation source which emits gamma rays. The thick detectors fix the location of impingement of a stopped gamma ray. By measurement of the energy released, the deflection angle .theta. occurring at the thin detector is determined from the Compton scattering equation.
Most of these imaging techniques require the measurement of the angle of the emitted or transmitted particles, i.e. X-rays or photons so that one can not only detect the energy but the direction from which the energy came. This angle must be determined so that projection functions can be used to map the object. It is this direction dimension that generally requires movement of the detector or the source with respect to the object to be imaged.
Despite such advances, a need exists for an efficient and precise medical X-ray detector or radionuclide detector having high detection efficiency (and thus low doses) excellent energy resolution (to eliminate background from scattered gamma-rays), excellent direction and position resolutions (to pinpoint tumors), and one which is simple, reliable and easy to use.
Ideally, the device should be able to image the distribution of two or more tracers (x-, or gamma and positron-emitters) simultaneously, thereby permitting:
i. the spatio-temporal correlation of physiological and anatomical processes delineated by the different tracers. PA1 ii. the spatio-temporal correlation of two or more physiological processes using different labeled compounds administered simultaneously. PA1 iii. the spatial correlation of two or more anatomical structures using different labeled compounds administered simultaneously.
Present nuclear medicine imaging devices are limited to tracers in the energy region below 511 keV. Great benefit would be obtained if devices were available in which the upper range of imagable energy could be increased, as it would then be possible to image a number of biologically important elements. These include, but are not limited to, Na-24, K-42, Mg-28, Ca-47, Fe-59, Co-58, and -60, and Zn-65.
Present nuclear medicine imaging devices are limited in size and can only cover a small portion of the body. A detector which could cover the entire body surface which could be commercially realized would have an important role in medical diagnosis and radiation protection programs (for example, whole body counting to determine body burdens of accidentally ingested or in vivo activated radionuclides). Such a system could also play a significant role in body composition analysis, i.e., naturally occurring high energy radiations from K-40 could be quantitatively imaged, thereby providing a measure of regional lean body mass, which can not be measured with presently available systems. In addition, the amount of Ca-49, and N-15 induced following neutron activation for bone and protein mass determinations, could be established at much lower patient dose than is now possible.
Current concern regarding the potential hazards of ionizing radiation exposures requires continued attention to ways of decreasing radiation exposure while not compromising the diagnostic information needed for proper patient management. A device which is able to increase the fidelity of image data, while decreasing the dose to the patient, would be an important addition to presently available medical imaging technology.